it can be concluded that the increase in cow productivity
results in a decrease in CH4 emission per kg milk, due to
cow nutrition in present dairy systems. However, it should
be noted that CH4 emissions during a cow career should be
split between milk and meat productions. The meat pro-duced should take into account not just the cow but also
that from the (male) offspring. New models that include
these factors need to be developed to better evaluate the
most environmentally efficient type of cow for a given
production system.
Intensification of livestock production through better
breeding and/or feeding to decrease GHG emissions needs
to be carefully assessed and will remain a hot debate in the
foreseeable future. The society and producers’ requests in
terms of welfare and health of animals, environment and
economic viability are sometimes contrasting and have to
be globally considered (Gill et al., 2009). For instance,
reduction in CH4 emissions in intensive systems of pro-duction could be offset by the potential negative con-sequences of using high concentrate diets on animal health
(e.g. acidosis) and, thus, farm profitability. Grain utilisation
in ruminant feeding risk needs also to be more critical with
the increased needs of grains for human consumption.
In the future, ruminants should still play a key role in the
valorisation of land under pasture. Furthermore, intensifi-cation of ruminant production as a CH4 mitigation strategy
requires a complete evaluation in terms of total GHG
emissions at the farm scale.
Variations in total GHG emission
Strategies for mitigation of CH4 enteric emissions will be
recommended, independently of the cost, only if they do
not result in an increase in the emission of other GHGs such
as CO2 and N2O. When additives or lipid supplementation
are used to decrease enteric CH4, it can be thought that
their use does not modify to a large extent the emission of
non-enteric CH4 and that of other GHG related to animal
production. On the contrary, the change in production system
(e.g. from a forage-based to a concentrate-based system
and/or from low-producing animals to high-producing
animals) results in simultaneous variation of all GHG. A
well-known demonstration had been made for dairy cows
by Johnson et al. (2000), who compared a grass system
with low-producing cows to a winter feeding system based
on concentrates with high-yielding cows. This latter system
produced 37% less enteric CH4 than the first one, but this
difference was compensated for by a much higher CH4
emission from slurry, compared to the very low emission
from urine and faeces on pasture.
To take into account all GHG emissions related to live-stock farming systems, different methods are used, either
derived from the life cycle assessment technique or using
farm-scale dynamic models. Integrative national and supra-national models are not described in this paper. Coefficients
of the different equations of the models may originate
from Intergovernmental Panel on Climate Change (IPCC)